10. Magic Bounce: Though this ability has seen better days in previous generations, it's still something most Pokemon can only dream of having. The main reason I've ranked it lower than I would have in previous generations is the addition of heavy duty boots, which generally decreases the influence of hazards. Additionally, the widespread distribution of Defog dramatically reduces the need for hazard prevention and also puts a damper on the impact of bouncing those hazards back to your opponent. Still, the immunity to other status moves is still incredibly useful.
9. Beast Boost: This ability is Moxie on crack. The thing that makes this ability better than Chilling Neigh, Grim Neigh, Soul Heart, or Moxie is that the stat boost can be targeted through EV's, IV's, nature, etc., to ensure that the appropriate stat is being boosted. There are a number of applications for this, generally all gearing a sweeping threat towards either snowballing out of control or becoming increasingly difficult to revenge kill through improving its speed or bulk.
8. Corrosion: This might be controversial but I think this ability could be amazing if provided to the correct Pokemon. Salazzle has a tough time pulling it off due to its Fire typing already forcing out most Steel-types and a number of Poison-types as well. Giving this to a Pokemon with more defensive stats and a typing which would otherwise encourage Steel-types to move in, such as Grass/Poison, could give that mon the tools to be a stall nightmare for bulky offense teams. In particular, Steel-types would become much easier to wear down, and this ability would prevent nearly any Pokemon, aside from those with a small list of select abilities, from being able to switch in on a Toxic.
7. Huge Power / Pure Power: This boosts a good number of underwhelming physical attackers into a position where they are legitimately viable in upper tiers. There's really not a huge need to go into detail about these imo.
6. Contrary: This could be truly diabolical if paired with the correct Pokemon. Imagine a user of a move like Draco Meteor, Overheat or V-Create with this ability. Of course the previous users running things like Superpower or Leaf Storm isn't bad, but getting boosts on moves with fewer potential switch-ins could really have devastating effects.
5. Ice Scales: This is of course the special equivalent to Fur Coat, which is missing from this list. I would rank Ice Scales higher due to the abundance of borderline broken special setup moves throughout Gen 8 singles like Geomancy and Quiver Dance along with the number of existing countermeasures to physical wallbreakers and sweepers including Intimidate, Rocky Helmet, contact-based abilities like Static or Iron Barbs, and the burn status. Special attackers run amok through OU in particular, and having this type of ability on a more competent Pokemon than the awful Frosmoth would likely achieve great success in OU.
4. Magic Guard: This is the better of the two magic abilities, as it provides immunity to the myriad sources of passive damage. This notably helps with hazards without having to use HDB for your item along with weather chip, Leech Seed, burn, poison, and partial trapping moves which have seen an uptick. I considered ranking this above Regenerator, but I think that it doesn't quite provide the same boost in defensive viability that Regen does. Still a really amazing ability either way, though.
3. Regenerator: Though this post doesn't do this, if you took into account the Pokemon that have each ability then this is by leaps and miles the best ability in OU. However, ignoring this it's still a great ability. Alongside HDB it allows for mostly drawback-free switching and can help offset the damage taken from pivoting into your opponent's attacks. While most slower Pokemon would be crippled by hanging on by a tiny sliver of HP, Regenerator can bring them back for another round. It's a metagame defining ability and it's not hard to see why since it can take a Pokemon that would be totally unviable without it and bring it into the top tier of a metagame.
2. Tinted Lens: This doesn't get a ton of credit given all that learns it are weak Bug-types and Noctowl. If you gave this to a stronger Pokemon with better offensive typing, it'd shred through the OU metagame (and possibly Ubers). The problem with existing users of the ability is that they're easily revenge killed and often still miss out on some important coverage. The ability is better for taking out weaker resists that would otherwise halt a sweep, but an effective user of this ability would need more power to muscle past neutral targets. Imagine this on something like Heracross or Volcarona.
1. Wonder Guard: It seems disingenuous to rank anything else here. Shedinja is a godawful Pokemon, but its ability is unquestionably the best in the game. Take a look at some of the cheese strategies on Youtube giving Wonder Guard to an Electric-type with a balloon. This is of course a rather terrible strategy as soon as your opponent figures out what you're doing, but it speaks to the power of this ability. If a halfway decent Pokemon gained this ability, it's hard to see it sticking around in OU for long before getting banned.
